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MQ-135.py
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MQ-135.py
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#!/usr/bin/python
"""
@name: MQ-135.py - MQ-135 GAS SENSOR
@disclaimer: Copyright 2017, KRIPT4
@lastrelease: Dic 27 2017 16:50
"""
"""
MQ135 gas sensor | ADS1115 (Analog-to-Digital Converter for Raspberry Pi)
Datasheet can be found here: https://www.olimex.com/Products/Components/Sensors/SNS-MQ135/resources/SNS-MQ135.pdf
Application
They are used in air quality control equipments for buildings/offices, are suitable for detecting of NH3, NOx, alcohol, Benzene, smoke, CO2, etc
Original creator of this library: https://github.com/GeorgK/MQ135
"""
import sys
import math
import operator
t = 22 # assume current temperature. Recommended to measure with DHT22
h = 65 # assume current humidity. Recommended to measure with DHT22
"""
First version of an RaspBerryPi Library for the MQ135 gas sensor
TODO: Review the correction factor calculation. This currently relies on
the datasheet but the information there seems to be wrong.
"""
# The load resistance on the board
RLOAD = 10.0
# Calibration resistance at atmospheric CO2 level
RZERO = 76.63
# Parameters for calculating ppm of CO2 from sensor resistance
PARA = 116.6020682
PARB = 2.769034857
# Parameters to model temperature and humidity dependence
CORA = 0.00035
CORB = 0.02718
CORC = 1.39538
CORD = 0.0018
CORE = -0.003333333
CORF = -0.001923077
CORG = 1.130128205
# Atmospheric CO2 level for calibration purposes
ATMOCO2 = 397.13
"""
@brief Get the correction factor to correct for temperature and humidity
@param[in] t The ambient air temperature
@param[in] h The relative humidity
@return The calculated correction factor
"""
def getCorrectionFactor(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG):
# Linearization of the temperature dependency curve under and above 20 degree C
# below 20degC: fact = a * t * t - b * t - (h - 33) * d
# above 20degC: fact = a * t + b * h + c
# this assumes a linear dependency on humidity
if t < 20:
return CORA * t * t - CORB * t + CORC - (h-33.)*CORD
else:
return CORE * t + CORF * h + CORG
"""
@brief Get the resistance of the sensor, ie. the measurement value
@return The sensor resistance in kOhm
"""
def getResistance(value_pin,RLOAD):
return ((1023./value_pin) - 1.)*RLOAD
"""
@brief Get the resistance of the sensor, ie. the measurement value corrected
for temp/hum
@param[in] t The ambient air temperature
@param[in] h The relative humidity
@return The corrected sensor resistance kOhm
"""
def getCorrectedResistance(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG,value_pin,RLOAD):
return getResistance(value_pin,RLOAD) / getCorrectionFactor(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG)
"""
@brief Get the ppm of CO2 sensed (assuming only CO2 in the air)
@return The ppm of CO2 in the air
"""
def getPPM(PARA,RZERO,PARB,value_pin,RLOAD):
return PARA * math.pow((getResistance(value_pin,RLOAD)/RZERO), -PARB)
"""
@brief Get the ppm of CO2 sensed (assuming only CO2 in the air), corrected
for temp/hum
@param[in] t The ambient air temperature
@param[in] h The relative humidity
@return The ppm of CO2 in the air
"""
def getCorrectedPPM(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG,value_pin,RLOAD,PARA,RZERO,PARB):
return PARA * math.pow((getCorrectedResistance(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG,value_pin,RLOAD)/RZERO), -PARB)
"""
@brief Get the resistance RZero of the sensor for calibration purposes
@return The sensor resistance RZero in kOhm
"""
def getRZero(value_pin,RLOAD,ATMOCO2,PARA,PARB):
return getResistance(value_pin,RLOAD) * math.pow((ATMOCO2/PARA), (1./PARB))
"""
@brief Get the corrected resistance RZero of the sensor for calibration
purposes
@param[in] t The ambient air temperature
@param[in] h The relative humidity
@return The corrected sensor resistance RZero in kOhm
"""
def getCorrectedRZero(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG,value_pin,RLOAD,ATMOCO2,PARA,PARB):
return getCorrectedResistance(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG,value_pin,RLOAD) * math.pow((ATMOCO2/PARA), (1./PARB))
"""
Re-maps a number from one range to another. That is, a value of fromLow would get mapped to toLow,
a value of fromHigh to toHigh, values in-between to values in-between, etc.
# Arduino: (0 a 1023)
# Raspberry Pi: (0 a 26690)
More Info: https://www.arduino.cc/reference/en/language/functions/math/map/
"""
def map(x,in_min,in_max,out_min,out_max):
return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min
def main():
value_ads = 3300 # value obtained by ADS1115
value_pin = map((value_ads - 565), 0, 26690, 0, 1023) # 565 / 535 fix value
rzero = getRZero(value_pin,RLOAD,ATMOCO2,PARA,PARB)
correctedRZero = getCorrectedRZero(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG,value_pin,RLOAD,ATMOCO2,PARA,PARB)
resistance = getResistance(value_pin,RLOAD)
ppm = getPPM(PARA,RZERO,PARB,value_pin,RLOAD)
correctedPPM = getCorrectedPPM(t,h,CORA,CORB,CORC,CORD,CORE,CORF,CORG,value_pin,RLOAD,PARA,RZERO,PARB)
print("\n MQ135 Gas Sensor:\n")
print("\t MQ135 RZero: %s" % round(rzero))
print("\t Corrected RZero: %s" % round(correctedRZero))
print("\t Resistance: %s" % round(resistance))
print("\t PPM: %s" % round(ppm))
print("\t Corrected PPM: %s ppm" % round(correctedPPM))
if __name__ == "__main__":
main()